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Brain Bender I am the layer of the earth that is responsible for moving the tectonic plates. The temperature differential between the top and bottom of my layer causes convection. Which layer of the earth am I? Plate Tectonics What is causing plates to move? Plate Tectonics Plate Tectonics: A model of global tectonics that suggests that the outer layer of the Earth, the lithosphere, is composed of several rigid, large plates that move relative to one another by sliding on a weak layer, the asthenosphere in the upper mantle; continents and ocean basins are passive riders on these plates. This is a unifying concept of geology developed after WWII and only accepted during the 1960s! Some Specifics on Plates Plates are composed of fragments of the lithosphere (crust and upper mantle) Plates may include both continental and oceanic crust Plates move in relation to each other at varied rates No major tectonic movements within plates Dynamic actions concentrated along plate boundaries Plate Boundaries Plate Boundaries Three types of plate boundaries: Convergent: plates collide, resulting in subduction and mountain building Divergent: plates moving apart, resulting in new lithosphere produced at mid-oceanic ridge Transform: two plates slide past one another Remember: Earth’s interior convection is mechanism for plate tectonics Convection in the Earth Convergent Boundaries Plates move together (converge) One plate may be forced beneath another Subduction: Process in which one lithospheric plate descends beneath another Subduction Zone: Convergence of tectonic plates where one plate dives beneath another and is consumed in the mantle 3 possible scenarios: Ocean to Ocean Ocean to Continent Continent to Continent Convergent Boundaries Ocean-Ocean Convergence Oceanic plate meets oceanic plate Denser plate forced under (Subduction) Trench forms Volcanism occurs in “Island Arc” Wide earthquake zones Examples: Japan, Philippines Oceanic-Oceanic Convergence Ocean-Continent Convergence Dense oceanic plate subducts beneath continent = subduction zone Melting produces magma = volcanism Arc of volcanoes forms on continent (volcanic mountain belts) Oceanic trench forms parallel to subduction zone Earthquakes Examples: Cascades, Andes Oceanic-Continental Convergence Name this specific type of boundary. Continent-Continent Convergence 2 continental plates meet Both are low density Neither can subduct (too buoyant) So-they build upward (as mountains) Shallow Earthquakes Example: Himalayas (India vs. Asia) Himalaya Mountains: India-Eurasia Collision Subduction of oceanic crust under Eurasia led to the collision of India with Eurasia, creating the Himalaya Mountains Continental-Continental Convergence Divergent Plate Boundaries Plates moving apart (extension) Occur in 2 places: Oceanic (Mid Ocean Ridges) Terrestrial (Continental Rift Valleys) Results in Creation of new seafloors Extensional stress and shallow earthquakes Basaltic volcanism Divergent Boundaries Continental Rift Continental divergent boundary. Example: East African Rift Zone Mid Ocean Ridge Oceanic divergent boundary. Example: MidAtlantic Ridge Transform Boundary Oceanic or continental crust Results in a shear zone between the plates. Minimal mountain building Minimal volcanic activity Lots of seismic activity! Example: San Andreas Fault, California Transform Boundaries Strike-slip faults (no vertical motion) Lateral motion (shear) Plates slide alongside one another Occur in 2 realms: Continents (such as the San Andreas fault) Mid ocean ridges where they offset ridge segments (fractures) San Andreas Fault Plate Boundaries Divergent plate boundary = midocean ridges Convergent plate boundaries = subduction zones Transform plate boundaries, San Andreas fault Plate Motion Plates move at variable speeds Max Velocity ~16-17 cm/yr Min Velocity ~ .1 cm/yr Average Velocity ~ 2.5 cm/yr Hot Spots Volcanic centers that originate from rising plumes of magma from the deep mantle, called mantle plumes Mantle Plumes: Long-lived structures in the mantle Originate at great depth, perhaps near the core-mantle boundary Results in Intra-plate (middle of plate) volcanism A chain of volcanoes forms as tectonic plates move over a stationary hot spot The bend of a seamount chain over a hot spot records the change of plate motion Mantle Mantle can behave as a solid or a fluid. Temperature, pressure, and stress are factors in how the mantle behaves Rapid stress causes the mantle to act as a solid Solid mantle can fracture and break Mantle Upper mantle is less fluid because it is cooler. Crust and upper mantle form the fractured lithosphere When plates move strain accumulates Too much strain causes the plates to rebound like a snapped elastic These movements are earthquakes Importance of Plate Tectonics The theory provides explanations for: Earth’s major surface processes The distribution of Quakes Volcanoes Mountains